Radioactive marking of the first draining lymph nodes (sentinel nodes) during an operation on malignant melanoma (skin cancer) and mammary carcinoma and some cancer types in the neck and abdominal regions has become a standard method in medical practice. Radioactively labelled tumour-specific pharmaceuticals are also gaining importance due to the high spatial resolution that can be expected when directly locating cancerous tissue. The quality and duration of the search for lymph nodes or tumours depends absolutely on the technical properties and handling of the gamma probes employed for locating the radioactively marked tissue regions.
The finding of radioactively marked lymph nodes with the aid of known gamma probes is usually carried out by means of a spatially-resolving activity measurement. Herein, the spatial allocation and therefore ultimately the locating of the lymph node is usually made in the region of the activity maximum by means of an imaginary projection of the probe centre into the tissue region. Depending on the probe diameter and the experience of the physician in using a particular probe type, this leads to subjective measuring errors which can make the finding of the node more difficult, delay it or, in extreme cases, even prevent it. Direct display of the tissue section being sought does not take place.